Lecture 11
Global Positioning System (GPS)
GPS
is network of satellites (24 total - 21 in use, 3 spares) that continuously
transmit coded information, which makes it possible to precisely identify
locations on earth by measuring distance from the satellites. GPS provides
continuous (round the clock), real time, 3-dimensional positioning, navigation
and timing worldwide in any weather condition. It was originally intended for
military applications, but in the 19805 it was made available for civilian use.
There are no subscription fees or setup charges to use GPS. Any person With a
GPS receiver can access the system, and it can be used for any application that
requires location coordinates.
Components of GPS
The
GPS system consists of three segments: 1) the segments: the GPS satellites
themselves, 2) the control system, and 3) the user segment, which includes both
military and civilian users and their CPS equipment. The space segment is
composed of the GPS satellites that transmit signals from space based on which
time and position of the user is measured. The whole set of satellite is called
‘constallation’ and also known as Global Navigation satellite system (GNSS).
There are three globally known GNSS. These are NAVSTAR (NAVigation System using
Timing and Ranging) of USA, GLONASS (Globalnaya Navigazionnaya Sputnikovaya Sistema,
or Global Navigation satellite System) of Russia, and GALILEO of European
Union. Indian Regional Navigation Satellite System (IRNSS) is an independent
regional navigation satellite system being developed by India. It is designed
to provide accurate position information service to user in India as well as in
the region extending up to 1500 km from its boundary, which is its primary
service area. The control segment consists of Monitor Stations, Ground Antennas
and a Master Control station (MCS). The monitor stations passively track all
satellites in view, accumulating ranging data. This information is processed at
the MCS to determine satellite orbits and to update each satellite’s navigation
message Updated information is transmitted to each satellite via the Ground
Antennas. The GPS User Segment consists of the GPS receivers and the user
community. GPS receivers convert satellite signal into position, velocity, and
time estimates. Four satellites are required to compute the four dimensions X,
Y, Z (position) and time. GPS receivers are used for navigation, positioning,
time dissemination, and other research. There are civilian applications for GPS
in almost every field, from surveying to transportation to natural resource
management to agriculture. Most civilian uses of GPS, however, fall into one of
four categories: navigation, surveying, mapping and timing.
Working Principles of GPS
Each
satellite transmits a message containing three pieces of information: the
satellite number; its position in space, and the time at which the message was
sent. The GPS receiver reads the message and saves the information. GPS
receivers take this information and use triangulation (otherwise called
trilateration) to calculate the user’s exact location.
A GPS receiver
must be locked on to the signal of at least three satellites to calculate a 2D
position (latitude and longitude) and track movement. With four or more
satellites in View, the receiver can determine the user’s 3D position unit can
calculate other information, such as speed, bearing, track, trip distance,
distance to destination, sunrise and sunset time and more.
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